What I'm seeing on Mastodon is a desire by quite a few people to use these open, public messaging systems as something more akin to private mailing lists. They have no desire for anyone-can-read/anyone-can-reply, they just want a nice little circle of people who all agree to be a part of the circle and to have the others there too.
I believe that many of the people who want this are too young to have ever been on old listserv-style mailing lists, and for them, this sort of messaging technology is obviously how you would do this sort of thing.
I don't think they are right, but I also see how they would think this, and they're not clearly wrong.